SRAM2 in Secure Zone Reset Issue with STM32U575 Using SBSFU

[Istillaga]

Hi,
I am working with SBSFU on a STM32U575 microcontroller, and I want to use the __attribute__((section(‘.ram2’))) to place a variable in the secure SRAM2 memory section so that the variable can write to it. However, the secure SRAM2 memory is reset every time the microcontroller is rebooted. I would like to know if it is possible to retain the data in secure SRAM2 permanently, and if so, would the linker script need to be modified to accommodate the use of this attribute?

[Jocelyn RICARD]

Hello @Istillaga ,

the SRAM2 content is erased on reset. This is a security feature. If you want to keep something in memory you should use other SRAM. Now if you still want to do that, you can change this behaviour in option bytes.

You will need to modify the SBSFU also because it checks this setting.

Best regards

Jocelyn

[Istillaga]

Hello @Jocelyn RICARD,

If I use another memory in the secure zone SRAM1 for example to be able to write using the __attribute__((section(‘.ram'))) I would have to modify the linker script right? how should I modify it?

[Jocelyn RICARD · Accepted solution]

Hello @Istillaga ,

If you need to write in secure you have the beginning of SRAM3 that is also secure

(You can check this in the region_defs.h :

#define  S_TOTAL_RAM_SIZE  (_SRAM2_SIZE_MAX + SRAM3_S_SIZE )

So, you will need to adapt your linker file to create a dedicated SRAM3 area

For instance, replace 

RAM (xrw): ORIGIN = S_DATA_START, LENGTH = S_DATA_SIZE

by

RAM (xrw): ORIGIN = S_DATA_START, LENGTH = S_DATA_SIZE - RETAIN_SIZE

RAM3_RETAIN(xrw): ORIGIN = S_DATA_START + S_DATA_SIZE - RETAIN_SIZE, LENGTH = RETAIN_SIZE

then create a section that points on RAM3_RETAIN with NOLOAD keyword.

Something like

.noinit (NOLOAD) :
    {
        . = ALIGN(4);
        *(.noinit)
        . = ALIGN(4);
    } > RAM3_RETAIN

Best regards

Jocelyn

[Istillaga]

Hello @Jocelyn RICARD,

I have made the changes and modified all the partitions so that the SRAM3 address, in my case 0x300D0000, is Non-Secure Callable, allowing me to write from secure_nsc. However, when I attempt to write to or read from the SRAM3 address, the system resets.

I have added the following code in secure_nsc.c:

__attribute__((section(“.noinit”))) uint32_t* ptr; 

    CMSE_NS_ENTRY void WRITE(void){
    
    ptr = (uint32_t*)0x300D0000;
    *ptr = 0x12345678;
    uint32_t value_read = *ptr;
}

[Istillaga]

Hello,

I made a mistake with the memory address which was SRAM3 0x30040000 with 0x300D0000. With 0x30040000 it works perfectly.